To help us in addressing bugs, the best way is to create an issue on the issue tracker. This lets us keep track each report separately and ensures it doesn't get lost.https://github.com/magarena/magarena/issues (requires github account)

For game play bugs, please include a gameplay report zip file. You can create a gameplay report by opening the options menu (press Esc during a duel) and selecting the "Gameplay Report" menu item.

Less preferred is to post your report in this thread. (requires forum account)